Graduate Research Course
Comparative Animal Sensory Biology
How 11 kingdoms of animals sense the world — compared modality by modality. Each module opens with a spectral atlas showing detection range across species, then compares the molecular and physical mechanisms side-by-side.
Hearing: Frequency Extent by Species
Audible frequency range, log-Hz axis — infrasound to ultrasound spans 6 decades.
Elephants and pigeons hear infrasound (<20 Hz). Bats, cetaceans, and mice access ultrasound (>20 kHz). The moth ear is narrowly tuned to the echolocation calls of its bat predator — a 60-million-year arms race.
Vision: Wavelength Extent by Species
Detected wavelength range, log-nm axis — UV through thermal IR, ≈2 decades.
Bees see UV that humans miss; pit vipers see thermal IR via the loreal pit organ (not photoreceptors — it is really a remote-bolometer thermoreceptor, shown here for comparison). The mantis shrimp has 16 photoreceptor types — more than any other animal — yet Thoen 2014 showed its colour discrimination is poorer than a pigeon’s.
Magnetoreception: Detection Threshold by Species
Minimum detectable field (log nT axis) — circles coloured by proposed mechanism. Earth’s field strip highlighted.
Most animals that navigate with geomagnetism operate within Earth’s 25-65 µT field strength. Two mechanisms compete: radical-pair cryptochromes in the retina (light-dependent, inclination-only compass) vs. magnetite-based receptors in beak or abdomen (polarity + intensity). Sea turtles use both, imprinting on the magnetic signature of their natal beach to return 30 years later.
Quantum-Mechanics Glimpse · Radical Pair
The bird compass is one of biology’s clearest quantum phenomena. A blue photon hits cryptochrome 4 (CRY4) in the retina and ejects an electron from FAD to a tryptophan chain, generating a spin-correlated radical pair [FAD•⁻…TrpH•⁺]. The pair is born in a pure singlet state and evolves coherently under the spin Hamiltonian
\[ \hat H = -\gamma_e\,\vec B\cdot(\vec S_1 + \vec S_2) + \sum_i \vec S_1 \cdot \mathbf A_i \cdot \vec I_i \]
where the first (Zeeman) term couples both electron spins to the Earth’s field B≈50 µT and the second (hyperfine) term couples spin 1 to nearby nitrogen nuclei. The two terms interfere, driving coherent singlet↔triplet oscillation at MHz frequencies. The ratio of singlet to triplet reaction products at the end of the coherence lifetime (~1 µs) depends on the angle between B and the radical-pair axis — giving an anisotropic chemical signal that is the compass output.
Hore & Mouritsen 2016 reviewed the theory; Xu 2021 (Nature) reported that purified CRY4 from the European robin shows measurably stronger magnetic-field sensitivity than CRY4 from non-migratory chickens — evidence that radical-pair biochemistry has been tuned by selection for navigation. See alsoMigration M2,QFT,Biophysics.
Featured Lectures
The Invisible Power of Animal Touch · M3: Vibration & Touch
6th Sense: Animals That Predict Natural Disasters · seismic P-wave detection, infrasound, ion-flux sensing
The “6th sense” in animals that predict earthquakes is not mystical — it combines Pacinian-corpuscle detection of Rayleigh P-waves arriving seconds-to-minutes before S-waves, infrasound from rock fracture, and possibly magnetic-field fluctuations and radon ionization. Elephants flee coasts before tsunamis; dogs react 30 seconds before humans feel tremors.
About This Course
Animal senses occupy different corners of the same physical reality: a moth hears sounds 10 octaves above a human, an elephant hears sounds 4 octaves below; a bee sees UV flowers invisible to us; a shark detects electric dipoles 6 orders of magnitude weaker than our finest voltmeter; a monarch butterfly reads the Earth’s 50-microtesla magnetic field; a snake images thermal infrared with a non-photoreceptor sensor. This course compares the molecular and physical mechanisms side-by-side.
Cross-links: Eagle (vision),Seal vibrissae,Cetacean sonar,Elephant infrasound,Magnetoreception,Emperor syrinx.
Nine Modules
M0
Foundations
Information theory of sensing, Shannon-limit SNR, Weber-Fechner scaling, labeled-line vs. population coding, GPCR vs. ionotropic vs. mechanotransduction.
M1
Vision Spectral Atlas
Compound/camera/mirror eyes, eagle fovea, mantis shrimp 16 photoreceptors, UV bees, tetrachromat birds, pit-viper thermal IR, opsin evolution.
M2
Hearing Spectral Atlas
Elephant infrasound vs. bat/dolphin ultrasound vs. oilbird sonar vs. owl asymmetric ears; cochlear tonotopy, Weberian ossicles, moth ear.
M3
Vibration & Touch
Star-nosed mole Eimer's organs, elephant foot Pacinian corpuscles, seal wake-tracking vibrissae, spider slit sensilla, Merkel/Ruffini/Pacinian, Piezo channels.
M4
Electroreception
Shark ampullae of Lorenzini (5 nV/cm), platypus bill, weakly electric fish EOD, honeybee electrostatic flower sensing, dolphin electroreception.
M5
Magnetoreception
Radical-pair cryptochrome (Ritz 2000, Xu 2021 CRY4) vs. magnetite-based; sea turtles, monarch butterflies, salmon, robin inclination compass.
M6
Chemoreception
OR gene counts (elephant 2000, dog 811, human 396), moth bombykol single-molecule, Jacobson's organ, bird olfaction, bee tarsal taste, fish amino-acid homing.
M7
Thermo & Hygroreception
Snake pit organs, TRP receptor evolution (TRPA1/M8/V1), Melanophila beetle IR fire detection, mosquito CO2+heat, insect hygroreceptors.
M8
Multimodal Integration
Optic tectum cross-modal neurons, bat/dolphin vision-echolocation fusion, predictive coding (Friston), avian wulst vs. mammalian cortex, attention gating.